If They Retrieve 10 Eggs, How Many Become Embryos?

The call comes a few hours after your retrieval. Someone tells you a number — nine eggs, fourteen, four — and then the conversation usually ends. The next update is a day away. The one after that is five or six days away.

Most people spend that gap doing arithmetic. If nine eggs were retrieved, how many embryos should there be? A friend got six blastocysts from twelve eggs — what does that say about your twelve?

What follows is an attempt to answer that honestly, using published figures rather than reassurance. It is not a prediction about any individual cycle, and it contains no claim about results at this or any other practice. It is what the research reports happens at each step — and, just as importantly, what it does not report.

Why you cannot multiply the percentages together

This is the first thing worth knowing, because it is the mistake almost every online "egg calculator" makes.

The figures below come from different studies, conducted on different groups of patients, in different countries, using different stimulation protocols, and measured against different denominators. One is a UK registry of 400,000 cycles. One is a Spanish clinic's genetic testing data. One is a Japanese program using minimal stimulation. Multiply a percentage from one against a percentage from another and you produce a number that no published source actually supports.

So read each stage as its own piece of information. The honest summary of the whole chain is that it narrows, that it narrows more with age, and that the width at the start matters less than people expect.

How many retrieved eggs are mature?

Not every egg collected is usable. Only eggs that have reached the stage embryologists call metaphase II can be fertilized.

The Vienna consensus — a joint reference document from the European Society of Human Reproduction and Embryology and Alpha Scientists in Reproductive Medicine, published in 2017 — gives a reference range of 75 to 90 percent of collected eggs being mature.

Two caveats matter here. This is measured in cycles using ICSI, where eggs are stripped of surrounding cells and individually assessed; in conventional insemination, maturity is not routinely graded, so your report may not mention it at all. And this is a reference range for laboratory performance, not a prediction for a person. A 2024 study of 563 treatment cycles found an average of 86 percent, but with wide variation between patients.

How many mature eggs fertilize?

Here the denominator matters more than the percentage, and it is the single most common source of confusion when people compare notes.

A clinic reporting "80 percent fertilization" may be counting fertilized eggs against mature eggs. Another reporting "58 percent" may be counting against all eggs retrieved. Those can describe the same cycle. If a number you have been given seems far off what someone else was told, this is usually why — it is worth asking which denominator was used.

The Vienna consensus sets a competency value of at least 65 percent and a benchmark of at least 80 percent of injected mature eggs fertilizing normally with ICSI, and at least 60 and 75 percent respectively for conventional insemination.

One finding here is worth stating plainly, because it runs against a common assumption. A randomized trial published in Nature Medicine in 2025, involving 824 patients without severe male factor infertility, found conventional insemination produced a slightly higher fertilization rate per egg retrieved than ICSI — 58.1 percent against 53.5 percent — with no meaningful difference in cumulative live births. ICSI is a solution to a specific problem. It is not a general upgrade.

How many fertilized eggs reach the blastocyst stage?

Almost all normally fertilized eggs begin dividing. The Vienna consensus puts cleavage above 95 percent, and a 2022 study at Brigham and Women's Hospital found cleavage rates between 96 and 100 percent that were independent of age entirely.

Reaching day five or six as a blastocyst is where the real narrowing happens. The Vienna consensus sets competency at 40 percent of normally fertilized eggs and a benchmark of 60 percent. That same 2022 study, following 746 cycles, observed blastocyst conversion ranging from 52 to 83 percent depending on the patient.

Age affects this step, but less than most people assume. One 2021 analysis reported blastocyst formation of 59 percent in patients aged 23 to 36, 54 percent at 37 to 43, and 48 percent at 41 to 43. The steep age effect arrives at the next step, not this one.

How many blastocysts are chromosomally normal?

This is where age does its work, and it is the step that explains most of what people find confusing about IVF.

The American Society for Reproductive Medicine's 2024 committee opinion on preimplantation genetic testing, citing the multicenter STAR trial, reports that 52 percent of embryos were aneuploid in patients under 35, rising to 64.5 percent in patients aged 35 to 40. Aneuploid means carrying the wrong number of chromosomes — the usual reason an embryo does not implant, or implants and then miscarries.

A 2017 study in Human Reproduction put the same thing the other way around: the proportion of blastocysts that are chromosomally normal falls from 57.4 percent at age 35 or under to 12.7 percent at 44.

And the 2022 cycle-level study found that euploid blastocyst conversion — normally fertilized eggs that become a chromosomally normal blastocyst — ranged from 28 percent down to zero across its age groups.

That last figure is worth sitting with, because it is the one nobody says out loud. A cycle that produces no transferable embryo is not a rare failure. At older ages it is a documented, expected outcome of a perfectly well-run cycle.

How often does a chromosomally normal embryo become a baby?

Roughly half the time. A 2022 study of 393 single euploid frozen transfers reported a 50.4 percent live birth rate, and review articles in the field converge on about the same figure.

There is an important qualification. It is sometimes said that once an embryo is known to be euploid, age no longer matters. The largest dataset on this — 8,175 euploid single embryo transfers, published in 2020 — found otherwise: implantation, pregnancy and live birth rates still decline with age even when only chromosomally normal embryos are transferred. Chromosome screening removes a great deal of the age effect. It does not remove all of it.

So how many eggs do you actually need?

The most useful answer comes from the largest study ever done on the question: an analysis of 400,135 treatment cycles from the UK national registry, published in Human Reproduction in 2011.

It found the live birth rate rose with the number of eggs retrieved up to about 15, plateaued between 15 and 20, and declined beyond 20. For patients with 15 eggs, the predicted live birth rate was 40 percent at ages 18 to 34, 36 percent at 35 to 37, 27 percent at 38 to 39, and 16 percent at 40 and over. The median number of eggs across the whole dataset was nine.

Two things about that decline past 20 eggs. It comes from an era when most transfers were fresh, and very high egg numbers are associated with ovarian hyperstimulation, which compromises a fresh transfer. It is not evidence that producing a lot of eggs is harmful in itself, and it does not apply in the same way to a freeze-all cycle.

For anyone weighing egg freezing, the same 2017 Human Reproduction study offers the clearest published guide: to reach a 75 percent likelihood of at least one live birth, the authors estimated a patient aged 34 would need about 10 eggs, one aged 37 about 20, and one aged 42 about 61.

What it means if your number was lower than you hoped

Three things are true at once, and holding all three is more useful than picking one.

The first is that egg count is a real variable and a low one is a genuine disadvantage. Nobody is served by pretending otherwise.

The second is that it is a far weaker predictor for an individual than it is across a population. The studies above describe averages over hundreds of thousands of cycles. Within any one of those age bands sit people who got one blastocyst from four eggs and people who got none from eighteen.

The third is the one that matters most practically: you need one. One chromosomally normal embryo that implants is a complete answer to the question. A cycle yielding fifteen eggs and a cycle yielding five can end in exactly the same place, and quite often do.

Where the drop happened matters more than how big it was

If a cycle did not go the way you hoped, the number that should interest you most is not the final one. It is which step lost the most.

A cycle where few eggs were mature raises questions about the trigger and its timing. A cycle where mature eggs did not fertilize raises questions about sperm and about the insemination method. A cycle where fertilized eggs stalled before day five raises questions about culture conditions and about egg quality. A cycle that produced good blastocysts, none of them euploid, is largely a question of age and chance, and is the hardest to change.

These point at different next steps. It is why a second opinion after a disappointing cycle should begin with the laboratory report rather than the outcome — and why "it didn't work" is not enough information to plan from. If you are reviewing a cycle done elsewhere, bring the embryology report, not just the result.

A closing note on comparing yourself to anyone

Nearly all of the figures above are drawn from populations, and nearly all of the comparisons people make are between individuals. Those are different things. A friend's six blastocysts from twelve eggs tells you about her cycle and nothing reliable about yours, in the same way that a clinic's published average tells you very little about what will happen to you — which is a subject we have written about separately in how we report our own results.

If you are waiting on a fertilization report tonight, none of this changes that number. But it may make the week easier to read, and it should make the conversation afterward a more useful one.

If you would like to go through a previous cycle's laboratory report with a physician, our initial consultation is free for all new patients. You can reach our office here or call 212-685-2229.

Sources

ESHRE Special Interest Group of Embryology and Alpha Scientists in Reproductive Medicine. "The Vienna consensus: report of an expert meeting on the development of ART laboratory performance indicators." Human Reproduction Open, 2017.

Berntsen S, et al. "IVF versus ICSI in patients without severe male factor infertility: a randomized clinical trial." Nature Medicine, 2025;31:1939–1948.

Gordon CE, et al. "Embryo attrition in planned PGT-A: predicting the number of available blastocysts for transfer." Journal of Assisted Reproduction and Genetics, 2022;39:173–181.

Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. "The use of preimplantation genetic testing for aneuploidy: a committee opinion (2024)." Fertility and Sterility, 2024;122:421–434.

Goldman RH, et al. "Predicting the likelihood of live birth for elective oocyte cryopreservation: a counseling tool for physicians and patients." Human Reproduction, 2017;32(4):853–859.

Reig A, et al. "The impact of age beyond ploidy: outcome data from 8175 euploid single embryo transfers." Journal of Assisted Reproduction and Genetics, 2020;37(3):595–602.

Sunkara SK, et al. "Association between the number of eggs and live birth in IVF treatment: an analysis of 400 135 treatment cycles." Human Reproduction, 2011;26(7):1768–1774.